1. Field of the Invention
The present invention relates to a chemical mechanical polishing slurry for semiconductor device planarization and, more particularly, to a chemical mechanical polishing slurry for use on polishing metal layers.
2. Background of the Invention
A semiconductor wafer typically includes a substrate, such as such as a silicon or gallium arsenide wafer, on which a plurality of integrated circuits have been formed. Integrated circuits are chemically and physically integrated into a substrate by patterning regions in the substrate and layers on the substrate. The layers are generally formed of various materials having either a conductive, insulating or semiconducting nature. In order for a device to have high yields, it is crucial to start with a flat semiconductor wafer and, as a result, it is often necessary to polish a side or part of a semiconductor wafer. If the process steps of device fabrication are performed on a wafer surface that is not uniform, various problems can occur which may results in a large number of inoperable devices. For example, in fabricating modern semiconductor integrated circuits, it is necessary to form conductive lines or similar structures above a previously formed structure. However, prior surface formation often leaves the top surface topography of a wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches and other similar types of surface irregularities. As a result, global planarization of such surfaces is necessary to ensure adequate focus depth during photolithography, as well as removing any irregularities and surface imperfections during the various stages of the fabrication process. Although several polishing techniques exist to ensure wafer surface planarity, processes employing chemical mechanical planarization or polishing techniques have achieved widespread usage to planarize the surface of wafers during the various stages of device fabrication in order to improve yield, performance and reliability. In general, chemical mechanical polishing ("CMP") involves the circular motion of a wafer under a controlled downward pressure with a polishing pad saturated with a conventional polishing slurry. For a more detailed explanation of chemical mechanical polishing, please see U.S. Pat. Nos. 4,671,851, 4,910,155 and 4,944,836, the specifications of which are incorporated herein by reference.
Typical polishing slurries available for CMP oxide processes contain an abrasive material such as silica or alumina in an acidic or basic solution. For example, U.S. Pat. No. 5,245,790 to Jerbic describes a technique for chemical mechanical polishing of semiconductor wafers using ultrasonic energy and a silica based slurry in a KOH solution. U.S. Pat. No. 5,244,534 to Yu et al. discloses a method of forming conductive plugs within an insulation layer. The process results in a plug of material, such as tungsten, which is more even with the insulation layer surface than that achieved using conventional plug formation techniques. Slurries of abrasive particles such as Al.sub.2 O.sub.3 and etchants such as H.sub.2 O.sub.2 and either KOH or NH.sub.4 OH are used in the first CMP step to remove the tungsten at a predictable rate while removing very little of the insulation. The second CMP step utilizes a slurry consisting of an abrasive material, such as aluminum oxide, and an oxidizing component of hydrogen peroxide and water. Similarly, U.S. Pat. No. 5,209,816 to Yu et al. teaches a CMP slurry comprising H.sub.3 PO.sub.4, H.sub.2 O.sub.2, H.sub.2 O and a solid abrasive material while U.S. Pat. Nos. 5,157,876 and 5,137,544 to Medellin teach stress free CMP agents for polishing semiconductor wafers which include a mixture of water, colloidal silica and bleach containing sodium hypochlorite. U.S. Pat. No. 4,956,313 to Cote et al. discloses a slurry consisting of Al.sub.2 O.sub.3 particulates, deionized water, a base and an oxidizing agent.
Since CMP has been successfully used to polish oxide surfaces for a number of years, a recent trend in the semiconductor industry is to utilize CMP techniques and slurries for polishing metal layers. However, even though some slurries and polishing techniques have been directed to metal layers, films and plugs, such as tungsten, aluminum and copper, chemical mechanical polishing of these metals for device fabrication has not been well understood or developed. As a result, the use of conventional silica or alumina slurries on metal layers has resulted in unacceptable polishing performance and yielded devices of poor quality. Accordingly, a need remains for improved chemical mechanical polishing techniques and slurries for the same which provide uniform metal layers, free from undesirable contaminants and surface imperfections.